Modern heat exchangers of the plate heat exchanger type are often provided with plates having a so-called herringbone pattern, i.e. a pattern which has indentations consisting of straight ridges and valleys. The ridges and valleys change their respective direction in the centre, producing the pattern that resembles a herringbone. In a stacked heat exchanger pack, alternate plates are turned by 180° so that the indentations cross one another. The thus stacked heat exchanger plates are brazed together, thus forming a compact and mechanically stable heat exchanger pack. Using the herringbone pattern of the heat exchanger plates, the resulting heat exchanger pack comprises a pattern of fluid channels through which the respective two fluids can flow and exchange their thermal energy.
When a heat exchanger pack of the afore-described type is exposed to pressure (in particular fluid pressure) and heat, the plates distort, causing a bending moment in the plates. In order to withstand high pressures, relatively thick metal sheets are used, e.g. with a thickness of 0.4 mm.
When such metal plates are pressed into the herringbone pattern, an unfavourable material flow takes place. If the press tool is not very accurately manufactured, cracks can appear in the plates. The relatively thick plates also require a high pressure in the press tool.
In a fully brazed heat exchanger, the joints are typically brazed with copper or a copper alloy solder placed between the plates. The copper (alloy) solder is frequently introduced as a coating of the metal sheets. The solder material collects at the crossing points of the indentations. The surface area and strength of the solderings are therefore quite small.
A fluid which is made to flow through a heat exchanger with a herringbone pattern is forced to flow over the ridges and down into the valleys. There are no unbroken straight flow-lines. At the leading edge of the ridges the flow rate is high, whereas the flow rate of the fluid is low behind the ridges (i.e. in the valleys). This variation in flow rate is very large. In the heat exchanger the heat transfer rate is high where the flow rate is high, but the heat transfer rate is low where the flow rate is low. A smaller variation in flow rate as it is the case in heat exchangers with a herringbone pattern is hence favourable.
When the flowing fluid contains two phases, i.e. the fluid is a mixture of a gas and a liquid, the recurring changes of direction at the ridges and valleys will have the effect that the gas forces the liquid away from contact with the plates. This reduction in wetting of the heat exchanger plates' surfaces also reduces the heat transfer rate.
The shape of the channels through a heat exchanger of the herringbone design also gives rise to a high pressure drop in the fluid as it passes through the heat exchanger. This pressure drop is proportional to the work done in forcing the fluid through the heat exchanger. A high pressure drop thus means high (mechanical) power consumption.
A heat exchanger trying to solve at least some of these problems is known from the document US 2007/0261829 A1. In this document it is suggested to provide a pattern on a heat exchanger plate that comprises indentations in the form of bulges and hollows, and between which channels are formed, passing through the heat exchanger. The shape of the thus formed channels gives rise to a moderate variation in flow rate through the heat exchanger, thereby resulting in a higher heat transfer rate. The thus formed heat exchanger plates are stacked together in a way that an upper plate is turned so that its downward-pointing hollows (bottoms) abut against the upward-pointing tops of a lower plate. The upper and lower plates are brazed together by forming solderings where the heat exchanger plates touch each other. However, it has been found, that these plates are prone to break in the side walls of the bulges during operation of the heat exchanger. Obviously, this seriously adversely affects the lifetime of the heat exchanger.